Abstract Background and purpose Intracoronary brachytherapy after percutaneous transluminal coronary angioplasty (PTCA) is usually performed with catheter-based treatment techniques in a straight vessel segment. There is a growing interest for treatment of bifurcations, which requires consecutive positioning of the source in main vessel and side branch. Materials and methods In-house developed software (IC-BT doseplan) is used to explore the optimal positioning of the source in modelled bifurcations with different shape for the source types available in our hospital, i.e. 90Sr/ 90Y, 32P and 192Ir. The results were summarised in look-up tables. The usefulness of these look-up tables was tested on various clinical examples. Results Tabulated results for the modelled bifurcations yield an estimation of the distance between the sources (gap width) in relation to the geometry and source type: 90Sr/ 90Y gap range 3–8.5 mm, 32P gap range 2–7 mm and 192Ir gap range 3.5–8 mm. The average dose relative to 2 mm from the source axis is: 90Sr/ 90Y, (mean±SD) 120±40%; 32P, 125±50% and 192Ir, 120±22%. The look-up tables also provide the coarse location and value of maximum and minimum dose: 90Sr/ 90Y, 220–60%, 32P, 230–55% and 192Ir, 170–85%. It appeared that the look-up tables provide a good approximation of the optimal gap width in the clinical examples. Conclusions Tabulated optimal gap widths are very useful for quick estimation of the required gap width for a given bifurcation and source type, in case the prescribed dose in both vessels is the same. In unfavourable geometries there is a risk of local underdosage. Individual treatment planning using a program such as IC-BT doseplan is then recommended.